# 99 questions/Solutions/16

(Difference between revisions)

(**) Drop every N'th element from a list.

dropEvery :: [a] -> Int -> [a]
dropEvery [] _ = []
dropEvery (x:xs) n = dropEvery' (x:xs) n 1 where
dropEvery' (x:xs) n i = (if (n `divides` i) then
[] else
[x])
++ (dropEvery' xs n (i+1))
dropEvery' [] _ _ = []
divides x y = y `mod` x == 0

An alternative iterative solution:

dropEvery :: [a] -> Int -> [a]
dropEvery list count = helper list count count
where helper [] _ _ = []
helper (x:xs) count 1 = helper xs count count
helper (x:xs) count n = x : (helper xs count (n - 1))

Yet another iterative solution which divides lists using Prelude:

dropEvery :: [a] -> Int -> [a]
dropEvery [] _ = []
dropEvery list count = (take (count-1) list) ++ dropEvery (drop count list) count

A similar approach using guards:

dropEvery :: [a] -> Int -> [a]
dropEvery xs n
| length xs < n = xs
| otherwise     = take (n-1) xs ++ dropEvery (drop n xs) n

Using zip:

dropEvery n = map snd . filter ((n/=) . fst) . zip (cycle [1..n])

Using take and drop:

dropEvery xs n
|n <= 0 = x
|x == [] = []
|otherwise = take (n-1) xs ++ dropEvery (drop n xs) n

A more complicated approach which first divides the input list into sublists that do not contain the nth element, and then concatenates the sublists to a result list (if not apparent: the author's a novice):

dropEvery :: [a] -> Int -> [a]
dropEvery [] _ = []
dropEvery xs n = concat (split n xs)
where
split _ [] = []
split n xs = fst splitted : split n ((safetail . snd) splitted)
where
splitted = splitAt (n-1) xs
safetail xs | null xs = []
| otherwise = tail xs